We're led to believe that Shakespeare used iambic pentameter for the dialogue of noble characters sense they were fully literate and commanded a more elegant way of speech, or at least that's what we draw from the times. A commoner would speak in black verse/prose since they aren't fully literate and don't have as good of a command of language. It was also a way to pick out those of lesser stance while reading/watching the play, so the audience would know what their place in society is to understand the play better.
